The present invention relates generally to a system and methods of cutting/shaping/machining bone material, taking into account its non-monolithic, composite nature, and more specifically to a system for the process and apparatus for laser assisted machining (“LAM”) of bone.
The cutting process forms one of the important bone shaping operations during orthopaedic surgeries. Today, although the orthopaedic surgery has come a long way through adaptation/integration of modern tools such as sensors and CAD (computer aided drawing) based generation of patient specific defined joint design and bone shaping/cutting parameters, it is still largely conducted by the surgeon using conventional tools such as saw, ultrasonic cutter, hammer, drill, etc. Such mostly conventional way of surgery is associated with human and tool attributes that mostly result in potentially increasing the risk of thermal damage (necrosis). This situation in turn leaves tremendous room for further development of operating tools and techniques. These further developments are likely to address several additional collateral adverse effects of orthopaedic surgery such as but not limited to (1) severe damage of tissues within and surrounding repaired/operated regions, (2) low precision in final dimensional tolerance on repaired/operated bone, (3) relatively slow surgical processes, (4) post-surgery tissue trauma, (5) rigorous pain, and (6) in some cases, post-surgery and related addition of cost.
Cutting saws and burrs are the traditional tools employed for bone cutting during orthopaedic surgery. Being a manual operation, it involves human errors and necessity of skilled surgeons thus making achievement of reproducibility difficult. Apart from these variabilities, there are other issues associated, such as thermally driven necrosis of tissues initiated by temperature rise due to extended period physical contact between the cutting/shaping tool and bone that leads to friction/abrasion between cutting tool and bone, along with heavy mechanical loading of bones during conventional mechanical cutting/shaping/machining. In general, cutting saw blades are much harsher than cutting burrs, with temperatures of bone rising above 100° C. The reason behind this increase in temperature is the large contact area of bone with the saw teeth. In addition, there are many cuts that need to be performed in order to shape the bone. Although the cutting operation using burr results in moderate temperatures (50-60° C.), burr cutting is limited to shallow cuts, thus not a complete substitute for the cutting saw.
To address the temperature rise, and avoid associated necrosis, many remedies have been explored that are mainly focused on (a) change in tool design, (b) improving method of operation and most prevalent (c) employing saline cooling. Out of these, (a), and (b) still require careful operational procedures to achieve lower heat generation. In case of (c), even though the temperature rise can be controlled, designing an effective cooling system becomes necessary. For cutting tools, internally cooled tools have been reported to be superior in terms of heat control than an external spray/mist cooling. Intricately designed tools and careful temperature and flow rate control are required to achieve low heat generation. Furthermore, due to the physical contact between the mechanical cutting tools and bone, a very cautious sterilization process for tools is required to avoid any risk of infection. Apart from these issues, the conventional bone cutting/shaping/machining also involves post tissue trauma and rigorous pain and long healing/recovery times.
Furthermore, as a result of the composite nature of the bone, the direction of mechanical loading during cutting critically determines the response of bone to cutting. Factors such as porosity, mineralization, and orientation-diameter-spacing of collagen fibers of histological structure of bone play a deterministic role in mechanical response. It has been well documented in the literature that the mechanical response changes drastically depending upon the orientation of these microstructural features with the loading direction. Thus anisotropy and heterogeneity of bone structure makes it difficult to predict the exact response of bone to cutting operation as well as leads to uneven site specific stress concentration and generation of microcracks and/or cracks. This situation also further leads to unpredictable fracture of the bone in ductile or brittle or mixed (ductile+brittle) manner that in turn leads to unpredictable cut surface quality (roughness) which holds tremendous bearing on post-surgery bone ingrowth and healing characteristics. In light of this situation it is extremely difficult to design/select cutting parameters (speed, force, feed rate) and cutting saw parameters (tooth spacing or pitch, tooth size, tooth form) for desired outcome and mostly remains an art related to skill and experience of the surgeon.